214
DOCS.
220,
221
MAY 1916
which
would
then
not
have been
enough
either.
I
have
already
sent
a
copy
to
Brouwer.[7]
Cordial
greetings, yours,
Einstein.
221. To
David Hilbert
[Berlin,] (13
Wittelsbacher
St.) Friday.
[25
May
1916]
Highly
esteemed
Colleague,
I
am
sitting
over
your
relativity
paper,[1]
which
I
must review in
Rubens’s
colloquium,
and
am
honestly toiling
over
it.[2]
I
do admire
your
method,
as
far
as
I
have
understood
it.
But
at
certain
points
I cannot
progress
and therefore ask
that
you
assist
me
with
brief
instructions.
I have been able to derive
your fine
formula
(6),
albeit
only
with the
aid of
the first form of
the
same on page
398.
By
contrast, I
have
not succeeded in
seeing directly
the
invariance
of
the
expression
P(J),
p.
399.[3]
How is
it
derived?
It
undoubtedly is
connected with the
following.
On
page
400,
slightly
below
the
middle
you
say:
“Now
dH/[dg]uv[k]l
is
a
mixed
tensor
...”[4] But
with
the best
of
my
efforts
I
see no
basis for
this
and for this
reason
do
not
understand the
following.
I
very
much
like
the
proof
on
page 403.[5]
I
still do not
grasp
the
energy principle
at
all,
not
even as
a
statement. In the
boundary
case
of the
vanishing
of
the
guv
derivatives,
for
ex.,
the
energy principle
is,
according
to
(18)
and the
gen.
form[6]
Edge'/dwl
= 0
in
the
form[7]
v
d(TlsPs)
^
dw
for
an
arbitrary
selection
of
ps.
From
this, however,
the
vanishing
of
not
only
the
EdTls/dwl’s
but also
of
the
Tls’s
themselves would
follow. How is
this
resolved?
Requesting
kind
enlightenment
and with
best
regards, yours,
A.
Einstein.